In this proposal, we propose to demonstrate and build a widely tunable, narrow linewidth, single frequency fiber laser by developing an innovative Er/Yb-co-doped single mode fiber. Such a fiber laser is needed for coherent lidar and interferometric fiber sensing. In Phase I, we will design and fabricate this new fiber, demonstrate fixed wavelength narrow linewidth single frequency fiber laser with linewidth of less than 3KHz, demonstrate wavelength tuning range of greater than 5nm, and demonstrate 5GHz frequency modulation with no any mode-hoping in less than 1ms. Successful demonstration of such a fiber laser will enable many new commercial and military applications.